docs: def frames(skip=1):
      '''Useful command to iterate on the trajectory frames. It can be
      used in a for loop.
  
      ::
      
          for i in frames():
              coords = current_trajectory()[i]
              # Do operation on coords
  
      You can use the option *skip* to take every i :sup:`th` frame.
      
      '''
      from PyQt4 import QtGui
      
      for i in range(0, viewer.traj_controls.max_index, skip):
          viewer.traj_controls.goto_frame(i)
          yield i
          QtGui.qApp.processEvents()